What could be better than a lion-sized carnivorous marsupial? If said marsupial hunted by stalking, then dragging its prey into the trees.
This possibility is discussed in an article by one of the many, excellent, bloggers over at ScienceBlogs.
According to Brian Switek, owner of the Laelaps blog:
A close relative of living koalas, kangaroos, and wombats, the largest species of Thylacoleo were lion-sized carnivores that stalked the Australian continent between 2 million and 45 thousand years ago.
Due to anatomical features both curious and incomplete, a debate has been raging over whether the hunting tactics of Thylacoleo were either similar to those used by leopards (who drag their prey into the trees) or to those used by lions (who don't). The evidence for either case would lie in the hind feet (if Thylacoleo climbed trees carrying heavy prey, the hind limbs would show obvious tree-climbing-facilitating specializations), of which, unfortunately, there were no complete examples.
The article describes the discovery of a complete hind limb, which shows major differences from those of cats.
The paleontologist explains:
Cats are digitigrade, or they are standing up on their "tippy toes" all the time. The foot of Thylacoleo, though, is plantigrade, or like ours in that the metatarsal bones support part of the foot that touches the ground (to pick an example among carnivores, bears also have plantigrade feet). The feet also have a bit of a curve to them; in the restored foot the bones around the ankle articulation slant to the left while the metatarsal and toes slant to the right.
The paper reported by the article comes down in favor of the Like-A-Leopard hypothesis, as the toes, the curvature of the foot, and the presence of retractable claws on the ends of the toes would all have aided the Thylacoleo to grasp tree trunks. More research is promised by the authors, and so evidence does seem to be accumulating to add another animal to the special group ensuring the elimination of the "Climbing a Tree" monster-avoidance strategy.